Subject: Re: netbsd machines get slow and hang, nfs suspected
To: Gordon W. Ross <firstname.lastname@example.org>
From: Charles M. Hannum <email@example.com>
Date: 03/26/1996 10:35:57
Interesting suggestion. That reminds me: The SunOS kernel has
wakeone(int wchan); /* wakeup just one process */
for use with things like widely used "mutex" locks, where we
don't care who gets to proceed, and we know only one process
will actually be allowed to proceed past the mutex.
It would be nice to identify the comon places where we are
prone to the "thundering herd" problem, and change them to
use using wakeone() instead of wakeup().
There are really a couple of different cases here:
1) tsleep() and wakeup() are being used for lock management. The real
distinction here is whether the process being woken up needs an
exclusive or non-exclusive lock. In this case, it should really be up
to the tsleep() caller what happens.
2) tsleep() and wakeup() are being used to handle resource depletion.
In this case, it's really the caller of wakeup() that knows how many
resources just became available.
It seems to me that a more flexible and less error-prone solution
would be to write `real' lock and resource management systems. These
aren't particularly complicated, and they would also help with
fine-grain MP locking...